A processing apparatus for performing plasma processing is used in manufacturing electronic devices such as semiconductor devices (see, e.g., U.S. Pat. Nos. 6,468,816 and 6,244,121). Each of processing apparatuses disclosed in U.S. Pat. Nos. 6,468,816 and 6,244,121 includes a processing chamber and a mounting table. The mounting table is provided in the processing chamber and holds a target object mounted thereon. In such a processing apparatus, the target object is transferred by a transfer unit to be mounted on the mounting table and processed by a plasma of a processing gas which is generated in the processing chamber. The transfer unit detects a transfer state by transferring a sensor device having the same shape as that of the target object.
However, in the processing apparatuses and the transfer units disclosed in U.S. Pat. Nos. 6,468,816 and 6,244,121, it is not possible to detect a position of the target object mounted on the mounting table, i.e., positional relation between the mounting table and the target object. Therefore, the transfer units disclosed in U.S. Pat. Nos. 6,468,816 and 6,244,121 cannot determine whether or not the target object has been mounted at a predetermined target position on the mounting table.
In the processing apparatus, a focus ring may be provided on the mounting table to surround a peripheral portion of the target object to improve in-plane uniformity of the processing of the target object. The focus ring is worn by the plasma and needs to be replaced. When the focus ring is replaced, an assembly error of the focus ring may be generated. When the assembly error of the focus ring is generated, the positional relation between the target object and the focus ring becomes different from that before the replacement of the focus ring even if the target object is mounted at the predetermined target position on the mounting table. Therefore, the in-plane uniformity of the processing of the target object may deteriorate.